1. Field of the Invention
The invention relates to clutch devices and more particularly to a lockup clutch of a hydrodynamic torque converter.
2. Description of the Related Art
EP 0 407 895 B1 discloses a lockup clutch for a hydrodynamic torque converter having an axially movable piston which can be pressed into a switching position via a friction facing at a contact face of a flange of the converter housing. The contact face is formed at a plate which is fastened to the side of the flange facing the piston and which carries the friction facing with which the piston enters a working connection.
During operation of a hydrodynamic torque converter, as a result of centrifugal forces there is higher pressure in the radial outer area than in the area situated further inward radially. Consequently, the piston of the lockup clutch is pressed against the friction facing with increasing radial magnitude and with higher force.
As a result of this, the carrying or supporting behavior is uneven along the radial extension of the friction facing, so that the area pressure occurring at the radial outer side can exceed the tolerance of the friction facing, without reaching the loading capacity of the radial inner region. This results in a reduction in the maximum torque which may be transmitted and increased wear of the friction facings.
This effect is intensified in that, while the lockup clutch is active, the piston is axially supported only in the radial area in which the friction surface is provided. Since this friction surface is often arranged in the outer radial area of the piston, the piston is axially loaded radially inside is area by the pressure in the converter housing without the ability to be supported at that location. Therefore, even a rigid piston will be elastically deformed, which also contributes to uneven support behavior at the friction facing and accordingly to increased wear or the friction facings.
DE 34 10 526 A1 shows the piston of a lockup clutch as a diaphragm-like structural component part which has a pressure plate in the area of extension of the friction facings. However, the above-mentioned fundamental problem cannot be solved in this way since, due to the elasticity of the diaphragm-like structural component part at the radial outside of the friction surface, a corresponding inclined orientation of the pressure plate relative to the axis of rotation and therefore an uneven loading of the friction facings is to be expected. Also, DE 44 25 912 A1 which likewise has an axially elastic piston does not offer a solution to the problem. Rather, it makes use of this problem in that the radial support surface of the friction facing at the converter housing is to be adapted depending on the respective pressure ratios in the torque converter.